Document processing jobs often require the printing of monochromatic sheets including black and white printed images, such as text, and sheets having color images, such as pictures, charts and other graphics. For example, the job may require that sheets with color pictures be inserted between sheets having black and white text images. In order to accommodate both black and white images and color images; document processing systems may incorporate at least one print engine for imparting black and at least one other separate print engine for imparting colors. Tightly integrated multi-engine printing systems such as those described in U.S. Pat. No. 7,136,616 could achieve this function using an integrated media path system.
In a tightly integrated document processing system, the sheets would typically be printed sequentially. If a sheet having a black and white image were printed followed by a sheet having a color image, the print engine for the color image would be cycled on to generate the color image. In this way, the sheet could be fed in a sheet processing stream and be positioned correctly within the output set of sheets. In a document processing job where the color sheets were relatively few, the color print engine would be turned on to generate one or a few sheets and then cycled down. The print engine would then be cycled up again when the next color sheet or sheets were needed. Accordingly, the color print engine may need to be cycled up and down many times during a processing job. An alternative option is to cycle up the color engine once and allow it to dead cycle while the monochrome prints are being printed. When dead cycling, the color engine is running but not producing prints.
Print engines, especially the expensive life limited photo receptor elements, are subject additional wear and tear upon each cycling up and down or any non-printing dead cycles. However, once a print engine is cycled on, a number of sheets may be processed with nominal wear on the print engine. In a document processing job where color sheets are infrequently dispersed through out the document, the color print engine must be cycled on and off frequently thereby leading to significant additional wear on the print engine resulting in a shortened effective photo receptor life and an increase run cost per printed page.
Document processing machines have been devised to address the high on/off cycling of the print engines. A number of sheets are processed at a time and then stored in a buffer unit. The buffer unit may be disposed adjacent a sheet inverting unit. A buffer unit may include a number of media paths where sheets can be stored and inserted into the processing steam as needed. However, such buffering devices require additional processing stations to be added to the document processing machine, thereby increasing the complexity, the cost and the footprint of the machine.
Accordingly, it would be desirable to provide a compact buffering system to reduce the wear on the print engines without adding additional footprint or significant complexity to the machine.